This invention relates to the manufacture of molded articles. The use of elastomeric materials such as synthetic rubber in the manufacture of molded articles such as shoe components has long been known. Such materials have high coefficients of friction and ground-gripping properties and are very flexible at low or ambient temperatures. With most types of rubber, however, it is necessary to compound them with vulcanizing agents and subject the compounds to mastication followed by molding operations in the presence of sufficient heat and for a time sufficient to effect vulcanization. Of course, for some end uses, other polymers such as polyvinylchoride may be employed. The latter type of polymer, however, exhibits certain undesirable characteristics for end uses such as footwear due for example, to high wet slip and cold stiffness.
In recent years a number of block polymers have been developed which are elastomeric and which exhibit the stress-strain properties of vulcanized rubber without having been chemically vulcanized. This special class of polymers is referred to as thermoplastic elastomers, since they not only exhibit typical elastomeric properties but also may be processed in equipment normally employed for the rocessing of ordinary thermoplastic polymers such as polystyrene processing polyolefins.
The most highly developed types of block copolymers include linear or branched species having thermoplastic polymer blocks interspersed with elastomeric polymer blocks. Typical examples of these include block copolymers of monoalkenyl arenes with conjugated dienes as well as certain hydrogenated derivatives thereof. Other suitable thermoplastic elastomers comprise olefinic block polymers having blocks of ethylene or propylene combined with olefin copolymer blocks. Typical commercial materials include those sold under the trade name "KRATON" by Shell Chemical Company and "KRATON G".sup.tm, sold by the same company. The latter block copolymers comprise polymer blocks of monoalkenyl arenes such as styrene combined with elastomeric polymer blocks of an olefin copolymer or a hydrogenated polymer block of a conjugated diene.
Suitable methods for the preparation of block copolymer thermoplastic elastomers may be found in the following patents:
U.s. pat. No. 3,265,765 PA1 U.s. pat. No. 3,333,024 PA1 U.s. pat. No. Re 27,145 PA1 U.s. pat. No. 3,594,452 PA1 U.s. pat. No. 3,726,944 PA1 U.s. pat. No. 3,244,664
Because of their desirable combination of physical properties, these thermoplastic elastomers have found ready acceptance in mumerous molding processes and particularly in the shoe industry as well as pharmaceutical, automotive and sporting equipment. In the past, the thermoplastic elastomers have generally been compounded with other components such as fillers, extenders, plasticizers and the like in a kneading-type of mixer such as a Banbury Mixer. Such mixers entail high power requirements and the compounded mixtures must thereafter be further treated to form nibs or pellets, which are then utilized in well-known molding machines such as injection molders and the like.
Another type of compounding process which is beginning to receive attention (see U.S. Pat. No. 3,877,101) is referred to as dryblending. By this is generally meant a low shear mixing process which blends the compounding ingredients together without the formation of a coherent mass and which results in a relatively free-flowing mixture of ingredients in particulate form--which forms a suitable feed for commercially available molding equipment.
Certain difficulties have been encountered in adapting dry blending to thermoplastic elastomer compounds wherein the compounds include additionally not only a plasticizer but also thermoplastic polymers such as polystyrene, polyethylene or polypropylene, and other known thermoplastic polymers. In many instances it has been found that dry blending has not accomplished the desired degree of dispersion of the thermoplastic polymer in the molded article. This may result in certain unsatisfactory physical properties in the end product.
In the present specification and claims, the term "thermoplastic polymer" is distinguished from the term "thermoplastic elastomer" or "thermoplastic rubber" in that the thermoplastic polymers are defined as non-elastomeric polymers, which have the generally well-known characteristics of ordinary thermoplastics such as those referred to hereinbefore, namely, polystyrene, polyethylene or polypropylene.
It is an object of this invention to provide an improved process for manufacturing molded articles utilizing combinations of thermoplastic rubbers, plasticizers and thermoplastic polymers. It is a further object of this invention to provide a process for utilizing a dry blending process for compounding these ingredients in which the subsequently molded article exhibits improved dispersion of the compound ingredients. Other objects will become apparent during the following detailed description of the invention.